How to set the SAP user password as unchangeable

Hello experts,
There are some group-sap-users in our System. I want to set the password of those group users as UNCHANGEABLE, so that they can't change their password themselves. The Users have the user type "dialog". By means of changing the user type to "service", the password can be protected. But then I have problem with the sap user license. Is there any other way?
Thanks in advance!

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